feat: Add FixedPrecision vault scale helpers and Open-zone deposit checks

New vaults created under V1.2 stay on a lifetime base grid so optional inflows cannot coarsen AssetsTotal, while pre-V1.2 vaults keep the dynamic scale they were created with.

include/xrpl/ledger/helpers/VaultHelpers.h

@@ -17,6 +17,69 @@ namespace xrpl {
 
 class STTx;
 
+/**
+ * Return the Vault's current live exponent.
+ *
+ * Legacy and CashBasis Vaults use the exponent of AssetsTotal. FixedPrecision
+ * Vaults floor that exponent at their lifetime base exponent.
+ */
+[[nodiscard]] int
+getVaultScale(SLE::const_ref vault);
+
+/**
+ * Return the Vault's base exponent.
+ *
+ * Legacy and CashBasis Vaults use their current live exponent. FixedPrecision
+ * Vaults use -Scale, or 0 for integral assets.
+ */
+[[nodiscard]] int
+getVaultBaseScale(SLE::const_ref vault);
+
+/**
+ * Return the Vault's posterior live exponent after applying an unrounded delta.
+ */
+[[nodiscard]] int
+getPosteriorVaultScale(SLE::const_ref vault, STAmount const& delta);
+
+/**
+ * Round an amount at the Vault's current live exponent.
+ *
+ * Reserved for LoanPay. Vault deposit, withdraw, and clawback round at the
+ * posterior live exponent instead.
+ */
+[[nodiscard]] STAmount
+roundToVaultScale(SLE::const_ref vault, STAmount const& amount, Number::RoundingMode roundingMode);
+
+/**
+ * Round an amount at the Vault's posterior live exponent.
+ */
+[[nodiscard]] STAmount
+roundToPosteriorVaultScale(
+    SLE::const_ref vault,
+    STAmount const& amount,
+    Number::RoundingMode roundingMode);
+
+/**
+ * Open-zone capacity ceiling: 9 * 10^(15 + baseScale).
+ *
+ * Defined only for FixedPrecision Vaults, where this is 9 * 10^(15 - P).
+ */
+[[nodiscard]] Number
+getVaultOpenLimit(SLE::const_ref vault);
+
+/**
+ * Check whether `amount` is an admissible optional inflow.
+ *
+ * Legacy and CashBasis Vaults always succeed. FixedPrecision Vaults must
+ * remain at their base scale after applying the rounded amount, and the
+ * posterior capacity (AssetsTotal + YieldUnrealized + rounded amount) must
+ * stay within the Open zone.
+ *
+ * The amount is rounded toward zero at the posterior live exponent.
+ */
+[[nodiscard]] TER
+checkOptionalVaultInflow(SLE::const_ref vault, STAmount const& amount);
+
 /**
  * From the perspective of a vault, return the number of shares to give
  * depositor when they offer a fixed amount of assets. Note, since shares are
@@ -53,9 +116,10 @@ sharesToAssetsDeposit(SLE::const_ref vault, SLE::const_ref issuance, STAmount co
  * Rounding strategy:
  * - Debits (withdrawals): Rounds down `|delta|` on the new scale to prevent
  *   paying out more than requested.
- * - Credits (deposits): Floors the resulting total asset balance and returns the
- *   difference from the current total. This prevents crediting the vault with
- *   more assets than the user deposited.
+ * - Legacy/CashBasis credits: Floors the resulting total asset balance and
+ *   returns the difference from the current total.
+ * - FixedPrecision credits: Rounds the delta toward zero at the posterior live
+ *   scale.
  *
  * Key rules:
  * - The returned magnitude never exceeds `|delta|`.

include/xrpl/protocol/Protocol.h

@@ -319,14 +319,20 @@ constexpr std::uint8_t kVaultDefaultIouScale = 6;
 constexpr std::uint8_t kVaultMaximumIouScale = 18;
 
 /**
- * Vault ledger-entry schema versions. Assigned to newly created
- * Vaults once featureLendingProtocolV1_1 is enabled. Vaults created before
- * activation are left without LEVersion (implicit legacy version 0,
+ * Maximum fixed-precision IOU scale factor for a Vault.
+ */
+constexpr std::uint8_t kVaultMaximumFixedIouScale = 10;
+
+/**
+ * Vault ledger-entry schema versions. Assigned to newly created Vaults by
+ * featureLendingProtocolV1_1 and later protocol amendments. Vaults created
+ * before activation are left without LEVersion (implicit legacy version 0,
  * accrual-basis accounting).
  */
 enum class VaultVersion : uint8_t {
     Legacy = 0,
     CashBasis,
+    FixedPrecision,
 };
 
 /**

include/xrpl/protocol/detail/features.macro

@@ -16,6 +16,7 @@
 // Keep it sorted in reverse chronological order.
 
 XRPL_FEATURE(SmartEscrow,                 Supported::No,  VoteBehavior::DefaultNo)
+// Requires LendingProtocolV1_1. New vaults take FixedPrecision plus cash-basis.
 XRPL_FEATURE(LendingProtocolV1_2,         Supported::No,  VoteBehavior::DefaultNo)
 XRPL_FIX    (Cleanup3_5_0,                Supported::Yes, VoteBehavior::DefaultNo)
 XRPL_FEATURE(ConfidentialMPTKeyRotation,  Supported::No,  VoteBehavior::DefaultNo)

include/xrpl/protocol/detail/ledger_entries.macro

@@ -513,6 +513,7 @@ LEDGER_ENTRY(ltVAULT, 0x0084, Vault, vault, ({
     {sfAssetsAvailable,      SoeDefault},
     {sfAssetsMaximum,        SoeDefault},
     {sfLossUnrealized,       SoeDefault},
+    {sfYieldUnrealized,      SoeDefault},
     {sfShareMPTID,           SoeRequired},
     {sfWithdrawalPolicy,     SoeRequired},
     {sfScale,                SoeDefault},

include/xrpl/protocol/detail/sfields.macro

@@ -239,6 +239,7 @@ TYPED_SFIELD(sfPrincipalRequested,       NUMBER,     14)
 TYPED_SFIELD(sfTotalValueOutstanding,    NUMBER,     15, SField::kSmdNeedsAsset | SField::kSmdDefault)
 TYPED_SFIELD(sfPeriodicPayment,          NUMBER,     16)
 TYPED_SFIELD(sfManagementFeeOutstanding, NUMBER,     17, SField::kSmdNeedsAsset | SField::kSmdDefault)
+TYPED_SFIELD(sfYieldUnrealized,          NUMBER,     18, SField::kSmdNeedsAsset | SField::kSmdDefault)
 
 // 32-bit signed (common)
 TYPED_SFIELD(sfLoanScale,                INT32,      1)

include/xrpl/protocol_autogen/ledger_entries/Vault.h

@@ -242,6 +242,30 @@ public:
         return this->sle_->isFieldPresent(sfLossUnrealized);
     }
 
+    /**
+     * @brief Get sfYieldUnrealized (SoeDefault)
+     * @return The field value, or std::nullopt if not present.
+     */
+    [[nodiscard]]
+    protocol_autogen::Optional<SF_NUMBER::type::value_type>
+    getYieldUnrealized() const
+    {
+        if (hasYieldUnrealized())
+            return this->sle_->at(sfYieldUnrealized);
+        return std::nullopt;
+    }
+
+    /**
+     * @brief Check if sfYieldUnrealized is present.
+     * @return True if the field is present, false otherwise.
+     */
+    [[nodiscard]]
+    bool
+    hasYieldUnrealized() const
+    {
+        return this->sle_->isFieldPresent(sfYieldUnrealized);
+    }
+
     /**
      * @brief Get sfShareMPTID (SoeRequired)
      * @return The field value.
@@ -571,6 +595,17 @@ public:
         return *this;
     }
 
+    /**
+     * @brief Set sfYieldUnrealized (SoeDefault)
+     * @return Reference to this builder for method chaining.
+     */
+    VaultBuilder&
+    setYieldUnrealized(std::decay_t<typename SF_NUMBER::type::value_type> const& value)
+    {
+        object_[sfYieldUnrealized] = value;
+        return *this;
+    }
+
     /**
      * @brief Set sfShareMPTID (SoeRequired)
      * @return Reference to this builder for method chaining.